In the case of one-to-one communication or one-to-multi communication in the traditional digital communication system, an application of Shannon's theory has a limitation in transmission capacity. Further, to receive a transmission signal without interference in transmission/reception, data needs to be transmitted based on a Nyquist rate to be able to perfectly recover the received signal without inter-symbol interference. Recently, in the case of satellite broadcasting and communication, a technology of improving spectral efficiency (S.E) compared to that of the existing technology has been proposed.
For example, technologies of increasing transmission capacity by adding a pre-distorter to a transmitter and an equalizer to a receiver in a digital video broadcasting (DVB)-satellite second generation (S2) modem structure or the DVB-S2 modem have been present in the past. However, the conventional technologies have a problem on how a minimum-mean square error (MMSE) equalizer is applied under the variable coding and modulation (VCM) or adaptive coding and modulation environment in which a frame length and a modulation scheme are changed in each frame unit. Further, the 42 tap MMSE equalizer has complexity of multiplication. Further, the existing MMSE adaptive equalizer is operated based on a data-aided (DA) mode using an appropriate training sequence (known data/pilot symbol) based on a simple least mean square (LMS) algorithm, but has a limitation in the application under the VCM/ACM environment.
Here, to improve the transmitting and receiving frequency efficiency or the spectral efficiency (S.E), the spectral efficiency per bandwidth may be generally improved by reducing a roll off (RO) of a baseband filter as represented by the following Equation 1. In the following Equation, RS represents a symbol rate, r represents a coding rate of a channel code, M represents a modulation order (for example, 4 for QPSK, 8 for 8PSK), and W represents a bandwidth, in which W=Rs*(1+R0) and as the roll off (RO) is reduced, the spectral efficiency is increased.S.E.=Rs×r×log2(M)/W  [Equation 1]
For example, to avoid a channel distortion in a channel in which a bandwidth of a transponder is about 100 MHz (−3 dB bandwidth), if it is assumed that the roll off (RO) is set to be 0.2, the distortion may be avoided when the transmission is made at a symbol rate of 83.33 MHz. However, when the transmission is made at the symbol rate of 83.33 MHz or more, the distortion occurs. Further, as the symbol rate is increased, a bandwidth of a carrier is increased and inter-symbol interference occurs due to a group delay characteristic, and therefore a need exists for a method for overcoming the above problem. Generally, the transponder is configured of an input multiplex (IMUX) filter, an amplifier, and an output multiplexer (OMUX) filter, in which since the IMUX filter has a larger bandwidth than that of OMUX filter, it is more important to compensate for a distortion of the OMUX filter than a distortion of the IMUX filter. A distortion of the amplifier is almost perfectly compensated by performing a pre-distortion when characteristics of the amplifier are known and therefore it is assumed that a non-linear distortion due to the amplifier is not almost present.
However, in the VCM/ACM environment, there is a need to detect a frame synchronization such as a start of frame and a pilot symbol, a modulation code rate (MODCOD), and the like; however, when the distortion, the inter-symbol interference, or the like is present, it is impossible to detect frame synchronization or the MODCOD. For example, when there are no inter-channel interference and additive white Gaussian noise (AWGN), as illustrated in FIG. 1A, normal 16 APSK (amplitude phase shift keying) transmission/reception is possible, but as illustrated in FIG. 1B, when even though the AWGN noise is not present, the inter-symbol interference is present, a signal to interference ratio (SIR) is reduced to 6.4 dB and thus constellation identification is difficult, such that it is impossible to detect the frame synchronization and the MODCOD.